1. Field of the Invention
The present invention relates to an imaging apparatus and a control method therefor. More particularly, the present invention relates to techniques suitable for use with an imaging apparatus having an autofocus function.
2. Description of the Related Art
Hitherto, electronic still cameras and video cameras have used, when performing autofocus control processing (hereinafter referred to as AF), methods for setting, as an in-focus position, a position of a focus lens, at which a high-frequency component of a luminance signal obtained from an image sensor such as a charge-coupled device (CCD) is maximized. The above methods include a known hill-climbing method that moves a focus lens in a direction in which a level of a high-frequency component (hereinafter referred to as a focus evaluation value) obtained from an image sensor increases, and that sets, as an in-focus position, a position of the focus lens, at which the level of a high-frequency component of a luminance signal obtained from an image sensor is maximized. A scan method is also known, which stores focus evaluation values while a focus lens is driven over the entire movable range thereof and which sets, as an in-focus position, a position of the focus lens that corresponds to a maximum one of the stored focus evaluation values.
There has been a method for performing, when such AF is performed, a focusing operation by dividing a movable range of a focus lens. Japanese Patent Application Laid-Open No. 2002-277726 discusses a method for performing focus adjustment by dividing a scan range into first and second regions and moving the lens in a normal still image photographing mode in one of the first and second regions, while the lens is moved in a specific operation mode in all the regions.
The above Japanese Patent Application Laid-Open No. 2002-277726 does not discuss a focus detection region (referred to also as an AF frame), in which a focus evaluation value is detected, in a photographing image plane in a focus adjustment apparatus proposed therein. However, there are various objects of varied sizes to be photographed. Accordingly, when the size of the AF frame is set to be constant for any object, the AF frame is too large for a small object. Thus, something, such as a background, other than the object may come into the AF frame. Consequently, a distance to the object may erroneously be measured. On the other hand, when the AF frame is too small for a large object, sufficient contrast is not obtained in the AF frame. Thus, a distance to the object may erroneously be measured.